The long-term goal of these studies is to understand the neural mechanisms of multijoint coordination as related to the control of stance. The general hypothesis is that the nervous system simplifies the problem of control by using "strategies" in which certain movement variables are actively constrained, or held constant. Further, these variables are likely to be higher level, mechanical variables such as joint torque, relative segment positions, etc. The problem of maintaining stance arises from the mechanical complexities of the musculo-skeletal system. There are more degrees of freedom in the many linked segments and their muscles than are required for the task. Moreover, dynamical interactions among the body segments are complex and not well-understood. The proposed experiments will examine a particular postural response in order to understand the underlying neural mechanism. From previous studies we found that when standing cats are translated in many directions in the horizontal plane they respond by applying force against the support surface in only two preferred directions, the "force constraint'. This constraint will be analyzed to determine whether it represents a neural strategy for stance control and how it might simplify the problem of control. The experiments will quantify postural reactions of awake, freely-standing cats in terms of kinematics, forces and EMGs. The specific aims are: 1. To determine whether the force constraint is present in animals naive to the support surface translations, or instead, develops with experience. 2. To establish whether the force constraint is an adaptive response to a predictable perturbation. 3. To investigate the 3 dimensional boundaries of the force constraint with regard to the requirements for both horizontal stability and vertical support. 4. To examine the force constraint boundary that is determined by forepaw-hindpaw separation, and to ascertain whether the force constraint is related to the need to stabilize the flexible spinal column. Results from these studies will increase our understanding of the normal mechanisms for controlling stance posture. This understanding is fundamental to the rational treatment of balance problems that are characteristic of many movement disorders, such as Parkinson's disease, cerebellar disease and spinal cord injury.